Arthroplasty involves the repair of a joint by replacing one or more portions of the joint to eliminate pain and improve movement. For example, loss of cartilage or friction between bone surfaces can be treated by inserting an artificial joint, which includes one or more prostheses designed to replace bone surfaces and cartilage while also allowing for a range of movement similar to the original joint.
Knee arthroplasty typically involves resecting (cutting away) the diseased and damaged surfaces of the lower end of the femur (thigh bone), the upper end of the tibia (shin bone), and the joint surface of the patella (knee cap). These surfaces are then replaced by artificial materials. The femoral component or prosthesis is typically made from a cobalt chrome alloy and is attached to the femur with fixation devices such as pegs, often with the use of bone cement to bond the femoral prosthesis to the underlying bone. The tibial component typically consists of two parts—a metal tray (titanium or cobalt chrome alloy) and a polyethylene insert—that are assembled together during surgery. The metal tray is fixed to bone with screws, pegs, or a stem; while the insert is locked into the metal tray and articulates with the femoral component.
The technical challenges in knee arthroplasty are: restoration of the natural alignment of the knee with respect to the hip and the ankle; regaining the range of motion of the knee; and inducing the artificially-implanted knee to move in a manner similar to a normal knee. These goals are accomplished by making the bone cuts at precise locations and orientation relative to the rest of the bone, selecting the appropriate size and shape of the prosthetic components, placing the prostheses at the appropriate location on the bones and with respect to each other, and selecting an insert of appropriate thickness such that the knee joint is neither too lose or too tight.
Despite continuous improvements in the design and manufacture of artificial joints and in surgical instruments, the actual arthroplasty relies primarily on the skill and expertise of the surgeon performing the procedure. Arthroplasty requires that a surgeon not only insert the artificial joint, but also “balance” the joint to ensure that the movement of the artificial joint is as similar as possible to a normal range of motion. Balancing the joint often requires careful measurement and cutting of bone, ligaments and other tissue, as well as load balancing to ensure that the force applied by the bones to the joint is evenly distributed and range of motion testing to determine if the artificial joint is capable of movement in the direction and distance required for normal movement. The balancing process often requires the surgeon to simply physically hold the joint and “feel” whether the movement of the joint and the forces being applied to the joint are correct. As a result, the process of balancing the joint is largely subjective, as it relies upon the experience and knowledge of the surgeon to understand whether the movement of the artificial joint is “about right.” Misalignment of any of these parameters may result in limited range of motion of the joint, continued pain at the joint and early failure of the artificial joint due to excess load distribution or friction.
To aid in balancing the artificial joint during arthroplasty, measurement devices have been developed which help a surgeon measure some parameters during the balancing of the joint. The most common balancing devices are mechanical in nature: the surgeon manually applies force on the device to distract the bones of the joints and the distance between the bones is visually measured. Some measurement devices incorporate sensors which can be inserted into the artificial joint to provide measurements about load distribution that are useful when attempting to balance the joint. Even with these measurement devices, the surgeon is still required to manually apply an unknown or inaccurate force to the joint in order to determine whether the joint is balanced. If the amount of applied force is inconsistent with the actual force applied to the joint during actual use, the joint may not move appropriately and may wear prematurely, leading to limited movement, pain, and eventual replacement or further surgical repairs.